Personal preference. Also, I just like Korean idols' visuals more than Japanese idols. Korean companies put a lot of emphasis on the physical appearance of their idols. That's why plastic surgery is popular in the Korean entertainment industry. Although I can't say that all the surgeries turn out well. Some of them do, while others... not so much.
Somewhat related, but some k-pop groups also do Japanese songs. Groups usually enter Japanese market after they've established a strong following in Korea, which is about 2-4 years after their debut. As for whether you consider this j-pop or not, it's really up to you. J-pop has a distinct sound which is difficult for me to explain, but I just know it when I hear it. For me, some of these songs can be considered as j-pop, while others are pretty much k-pop in the Japanese language.
What's your nationality? Do you understand Korean or Japanese? o:
